The invention relates to a heat exchanger for efficient transfer of heat between a thin film of primary fluid and a secondary fluid or vapour, and to a method of heating or cooling of liquids, especially those with high viscosity, which have a tendency to solidify on the heat-transmitting surface, and where scraping of said surface is essential for an optimum heat transfer.
Flowing of highly viscous liquids tends to be laminar, which means that most of the heat transferred to the fluid from a heat-transmitting surface will be conductive. Agitation can more or less alter the laminar flow to turbulent flow and thereby increase the convectional heat transmission. In very viscous liquids, it is very hard to create much of a turbulent flow within the confines of the heat exchanger, and thus most of the heat transfer will be conductive. In these cases it is essential that the layer of liquid be heated is as thin as possible.
To increase the heating area, some heat exchangers are built with double walls, such that the liquid flows between two heat-transferring walls. This obviously increases the efficiency of the heat exchanger.
Some liquids tend to solidify on the heating surface, thus retarding the conductive heat process by building an insulating layer of product on the heat-conducting wall. This obviously reduces the efficiency of the heat exchanger.
Installing means to scrape the heat conductive surface often solves this problem. The scraped heat exchangers which are described in the patent literature can de divided into three basic categories: (1) those with single or dual scraped surfaces; (2) those with rotating or linear scraper movement; and (3) those with product propelled or foreign propelled scrapers.
A scraped heat exchanger much similar to the one here described is disclosed by R. L. Smith in U.S. Pat. No. 3,430,928, where the scrapers are imbedded into a rotating inner shaft. The main differences are that the machine here described has the facility for scraping both surfaces, and the force applied to the scraper to enhance the scraping action comes from axial forces applied from the outside rather than the centrifugal forces applied to the scrapers in the cited patent.
Douglas W. P. Smith in his U.S. Pat. No. 5,228,503 describes a dual scraped surface heat exchanger where a helically-formed auger on which scrapers are mounted is located in the annulus between two stationary cylinders. As the viscous liquid is pumped into the annulus the flow will affect the helical auger and it will start rotating. The rotating helical auger has two purposes 1) to create a turbulent flow; and 2) to scrape the two surfaces by means of scrapers affixed to the helical auger.
U.S. Pat. No. 4,126,177 to Robert L. Smith describes a similar machine with an external power source to drive the helical auger. The disadvantage of both the inventions is that the annular space required for the auger and scrapers limits possibility for thin film fluid processing. This is particularly a problem with highly viscous products, such as licorice, which tends to behave as in laminar flow if not forcibly agitated. Thus to agitate such product will require a very rigid, and consequently a space consuming, agitator. With such liquids it is sometimes an advantage to maintain the laminar flow in a thin film while keeping the dual heat transmitting surfaces scraped. In such a system the proximity of the surfaces is essential to the transfer of heat by conductivity. In none of the cited inventions is this possible.